1. Field of the Invention
This invention relates to a method of recording and detecting servo information for positioning a magnetic head in a magnetic disk apparatus for recording and reproducing data.
2. Description of the Prior Art
In a magnetic disk apparatus or the like, a plurality of tracks are provided concentrically, and a magnetic head is positioned to correspond to a given track among these tracks for writing or reading data. With most conventional magnetic disk apparatuses, an exclusive data recording area and exclusive servo area for positioning information for the magnetic head are provided. Positioning information is preliminarily written on the exclusive area for servo positioning the magnetic head.
With the recent trend for smaller track pitch, a system is adopted, in which information for positioning the magnetic head is preliminarily written in the same recording area as for recording data, the positioning information for effecting the positioning according to servo information read out by the magnetic head itself, which reads and writes data. This method requires a technique of effecting the positioning of the magnetic head by separating servo information from data read out from the recording area. Such a technique is disclosed in, for instance, Japanese Patent Publication 58-501644 and Japanese Patent Publication 63-100675.
This prior art technique will now be described with reference to the drawings. FIG. 5 is a view showing a recording medium with data sectors and servo sectors provided alternately on the same area. Reference numeral 1 designates recording medium, 2 data sectors on medium 1 for recording data, and 3 servo sectors on medium 1 for recording servo information.
FIG. 6(a) is a view showing, to an enlarged scale, part of servo sector 3 shown in FIG. 5. The showing is simplified to facilitate the explanation of an embodiment disclosed in the Japanese Patent Publication 58-501644 noted above. Referring to the Figure, reference numeral 4 designates servo tracks, each of which consists of erasing gap 5 without magnetization inversion for a predetermined interval, and preamble, sector mark, track data and burst sections 6 to 9 provided in the mentioned order subsequent to erasing gap 5. Track data section 8 consists of track address 8a and clock check code 8b, and burst section 9 consists of A and B bursts 9a and 9b. Reference numeral 10 designates the magnetic head. Designated at X is the orbit of head 10. FIG. 6(b) shows magnetic inversion of orbit x of head 10 on the recording medium, and FIG. 6(c) shows a read-out signal when orbit X is traced by head 10.
FIG. 7 is a block diagram showing a prior art circuit for separating servo information. Magnetic head 10 scans recording medium 1 to read out magnetization inversion written on medium 1. A signal from head 10 is amplified by amplifier 11, which provides an output to additional amplifiers 12 and 13 and also to peak detector 14. Amplifiers 11 to 13 merely amplitude amplify the signal from head 10 for obtaining signal levels suitable for a subsequent electronic circuit. Peak detector 14 produces pulses corresponding to positive and negative peaks of the signal from head 10. The pulse output of peak detector 14 is supplied to data separator 15. Data separator 15 decodes the output of peak detector 14 to be "0" or "1" according to reading the code of servo information. The construction of data separator 15 thus depends on the recording code of servo information. Generally, data separator 15 requires a clock synchronized to the recording code of servo information. The synchronized clock is obtained by utilizing the preamble with consequent "0"s. The recording code can be readily decoded to "1" and "0" according to the synchronized clock and output of peak detector 14.
The output of data separator 15 consists of a series of data and is supplied to the input side of shift register 16. Data separator 15 also generates a synchronized clock, which is supplied as the clock input to 6-bit shift register 16 and is also supplied to 6-clock counter 17.
The output of amplifier 12 is sources to erasing gap detector 18, which is a circuit for starting a servo information recovery process. More specifically, by detecting that no signal is received from magnetic head 10, the erasing gap detector detects the erasing gap 5, and in response thereto generates a logic level "1" on line 19 indicating the "presence of erasing gap". The signal on line 19 is supplied, together with a "timer enable" signal supplied from timer 20 to line 21, to the input side of AND gate 22. If both the inputs to AND gate 22 are high, an "erasing gap enable" signal is generated on line 23 and used to operate sector mark decoder 24. Sector mark decoder 24 is connected to the output of shift register 16 such as to monitor the first 3 bits of data. Thus, sector mark decoder 24 provides a "start" pulse to line 25 when and only when there is an "erasing gap enable" signal on line 23 and the first three bits of shift register 16 are "111". The "start" pulse on line 25 is supplied to 6-clock counter 17. Erasing gap detector 18 consists of a general-purpose peak detector and a counter for checking whether there is a signal from head 10. Sector mark decoder 24 consists of a simple three-input AND gate for detecting three "1"s in shift register 16. When a "start" pulse is provided to line 25, 6-clock counter 17 counts six synchronized clock pulses from data separator 15, and at the time of counting it generates "an-end-of-counting" signal to line 26. The signal on line 26 is supplied to three-input AND gate 27. To AND gate 27 are supplied a "timer enable" signal on line 21 and the output of clock check decoder 28. Clock check decoder 28 is a similar circuit to sector mark decoder 24 and connected to the output side of shift register 16 such as to monitor the newest 2-bit data. More specifically, the output of clock check decoder 28 is high when shift register 16 receives track data 8 subsequent to "111" of sector mark 7 and also receives succeeding 2-bit clock check code 8b of track data 8. Thus, when three inputs to AND gate 27 appear simultaneously, this means that track data 8 is loaded accurately, and thus a "track address load" signal is generated on line 29. Microprocessor 30 reads out track address 8a from the output of shift register 16 as soon as it receives a "track address load" signal. The "track address load" signal is also coupled to a start terminal of timer 20, and timer 20 has several functions. When timer 20 receives a "track address load" signal, it supplies timing signal to each of sample/hold circuits 31 and 32 for A and B bursts 9a and 9b. The outputs of sample/hold circuits 31 and 32 are supplied to a servo circuit (not shown) and used for the positioning of the magnetic head. Timer 20 provides the "timer enable" signal noted before by determining a timing, at which next servo sector 2 appears. Further, it determines time until a "track address load" signal is received since the provision of a "timer enable" signal. If the determined time differs greatly from a predetermined time, it generates a "failure" signal on line 33 to prevent erroneous processing of the servo sector.
The track address and sample/hold signals of A and B bursts 9a and 9b, which are read out accurately as described above, are supplied to a servo circuit (not shown) for ordinary servo control.
According to the Japanese Patent Publication 58-501644 noted above, a gray code is written repeatedly three times in track address 8a, and 4 bits are set as clock check code 8b to minimize erroneous reading of track address 8a due to a defect in the medium.
However, if there is medium defect 34 in sector mark 7 as shown in FIG. 8, sector mark decoder 24 can not provide a "start" pulse, thus resulting in a "failure" state. More specifically, any conventional servo data includes a pulse for timing, and if such pulse fails to be detected due to a medium defect or like cause, a series of servo data becomes defective.